Antenna system for soot detecting

ABSTRACT

An apparatus for detecting the accumulation of particulate material on a filter medium formed of dielectric material and disposed in a chamber is provided. The apparatus is operable for generating and transmitting an RF signal through the filter medium and for monitoring the transmission loss of the signal through at least a portion of the filter medium so as to provide an indication of the content of particulate material accumulated on the filter medium. The apparatus includes an input antenna for transmitting the signal and having at least one antenna element extending longitudinally of the chamber and disposed within the filter medium; and an output antenna for receiving the signal transmitted by the input antenna and having at least one antenna element extending longitudinally of the chamber and disposed within the medium in parallel, spaced apart, axially overlapping relation with respect to the input antenna element.

The present invention relates, in general, to an apparatus for detectingthe concentration or level of accumulation of RF susceptible,particulate material on a filter medium and, more specifically, to anantenna system for use therein.

BACKGROUND OF THE INVENTION

As is well known, a filter is placed in the exhaust system of dieselengines to remove soot from the exhaust gases of the engine. The filtermust be changed or cleaned from time to time to ensure that sootaccumulations do not adversely affect engine operation. It is known toremove or incinerate the soot particles by subjecting the filter, insitu, to heat from a fuel burner or other heat generating device, orfrom suitable running of the engine. Incineration is to be performedwhen the accumulation has reached a level where further accumulationwould adversely affect engine performance or before that incinerationwould produce excessive temperatures and possibly damage the filter.There is a need, therefore, for a method and apparatus which monitorsthe level of soot accumulation and provides a signal when theaccumulation reaches a predetermined level.

Soot accumulations exhibit dielectric properties. Accordingly, it ispossible to monitor the level of soot accumulation on a diesel enginefilter medium by detecting changes in the effective dielectricproperties of the filter medium. The complex permittivity of a materialis comprised of two components: a real component called the "dielectricconstant" and an imaginary component called the "dielectric lossfactor". Changes in either of these components can be detected using RFinterrogation methods. It should be mentioned at this point that thedielectric constant and loss factor of soot increases with increasingtemperature. This affects both transmission and reflection (resonance)type of measurements.

One method of applying this concept to monitoring soot levels in dieselfilters is to construct the filter housing or containment in the form ofa RF waveguide and then periodically excite the waveguide with RF energyat a fixed frequency and measure the reflected power. The reflectedpower will be a function of soot accumulation on the filter. Morespecifically, for any RF system, it is usually possible to determine afrequency at which the electrical load, i.e. the filter medium, thediesel soot and the filter containment, represents a matched impedancewith respect to the power source. In other words, the equivalentelectrical resistance, capacitance and/or inductance of the load arematched to the RF power source. When the load impedance is perfectlymatched to the power source, all emitted RF power is absorbed by theload. If the impedance is not matched to the RF source, some of the RFpower will be reflected from the load. The degree of load mismatchdetermines the mount of reflected power and, hence, reflected power canbe used to measure the change in the effective dielectric constant. Thismethod can be generally referred to as a reflectance or resonance typeof measurement.

U. S. Pat. No. 4,477,771 granted to the General Motors Corporation onOct. 16, 1984 describes a method of detecting soot content in aparticulate trap using this method. The method detects changes in theeffective dielectric constant only. The patent provides a metal filterhousing constructed as cylindrical waveguide which defines a closed, RFresonance cavity for receiving a ceramic filter. A single probe ispositioned at one end of the cavity and behaves as both a transmittingand a receiving antenna. A reflective screen is positioned at theopposite end of the cavity. All connecting exhaust pipe diameters arebelow the cutoff diameter of a circular waveguide needed to transmit theRF energy at the frequencies used in the device. The probe is connectedto a RF source through a directional coupler and an isolator. A detectoris also connected to the probe through the directional coupler. In onemode of operation of the device, the RF source is operated at theresonant frequency of the cavity when the filter is loaded withparticulates to its maximum desired accumulation and the detector isoperated to detect a null condition in the reflected signal which occursat the resonant condition. Upon detecting such a condition, the detectorgenerates an output signal operable to effect operation of a lamp oralarm. In a second embodiment, the reflective screen is replaced by asecond probe positioned at the remote end of the cavity. One probe isconnected to the power source and the other probe is connected to thedetector.

There are a number of practical and technical problems with thisapproach. From a practical point of view, it is important to understandthat the commercial viability of a RF-based device depends on itscomponent count and, more on its component price. In this latterrespect, higher operating frequencies incur higher component andfabrication costs. The device also tends to display poor sensitivity andis prone to large measurement errors due to the effect of temperature onthe effective dielectric constant for reasons described below.

From a technical point of view, there are two factors which must beconsidered and which have been overlooked by the prior art. One factorrelates to the properties of the filter containment or housing and theother relates to the properties of soot. Dealing firstly with the filterhousing, based on wavelength considerations, there is a frequency belowwhich a waveguide will not allow RF energy to propagate withoutsignificant attenuation. The frequency below which this occurs is calledthe "cutoff frequency" for that waveguide geometry. The formula forcalculating this cutoff frequency and the attenuation for thetransmission of frequencies below cutoff is well known to thoseknowledgeable in the art. It can be shown, for example, that the cutofffrequency for a 14.4 cm diameter filter is greater than 1.2 GHz and fora 30.5 cm diameter filter, the cutoff frequency is greater than 0.5 GHz.If the filter containment is a cylindrical resonator and the frequencyfor the lowest mode (and frequency) for resonance is calculated, onefinds that for the smaller filter (14.4 cm Diameter×15.24 cm Long) theTE.sub. 111 resonant frequency is 1.6 GHz and for a TM₁₁₁ resonance, theresonant frequency is 1.9 GHz. Similarly, for a 28.6 cm Diameter×30.48cm long filter, the TE₁₁₁ resonant frequency is 0.79 GHz and, for theTM₁₁₁ mode, the frequency is 0.94 GHz. These calculations clearlyindicate that conditions for resonance require even higher frequenciesthan for transmission. These high frequencies result in high componentand fabrication costs.

The properties of soot (carbon particulates) also have a significantimpact on viability of RF-based measurement methods. Soot is aparticularly lossy dielectric and it is for this reason that carbonblack (soot) is added to materials like rubber to increase the carrier'sability to be heated in a RF field. Terminal loads for RF systems arealso constructed of carbon. The dielectric constant of the soot changeswith temperature and hence the effective dielectric constant of thefilter changes with temperature. This means that the resonant frequencyshifts with changes in both soot accumulation and temperature. Clearly,this effect must be accounted for in measuring soot accumulation in afilter heated by hot diesel exhaust. This factor adds to the complexityand cost of the device.

For either of the methods proposed in the above described patent, themetal housing containing the empty filter must act as a narrow bandpassRF filter in order to make the measurements described in the patent.That is to say, the resonant cavity thus formed should allow energy toenter the cavity over only a very narrow range on either side of theresonant frequency and reject or reflect RF energy at all otherfrequencies (i.e., a narrow bandpass RF filter). Unfortunately for themethods described in the patent, the accumulation of soot not onlychanges the effective dielectric constant of the filter, therebyshifting the resonant frequency of the cavity, but it is also causes thecavity to increase its bandpass frequency range due to the effects ofthe very high dielectric loss factor of the soot, a factor notconsidered in the patent. In fact, above a range of soot load andtemperature combination, the soot becomes a purely resistive load over awide range of frequencies (i.e., it becomes a broadband terminal carbonload). When the load becomes mainly resistive in nature, reflectionsdrops virtually to zero. Since this phenomenon is broadband in nature,it is no longer possible to measure a resonant frequency (i.e., there isno difference in the amount of power being reflected over a wide rangeof frequencies).

In summary, reflectance or resonance type measurements of the typedescribed in the above mentioned patent arc precluded from usingfrequencies below the resonant frequency defined by the geometry of eachfilter and/or its metal containment. There is a manufacturing costpenalty associated with the relatively high frequencies that must beused by these methods. The high loss factor of the soot, as determinedby soot concentration, temperature and RF frequency, places severerestrictions on the range of soot concentration that can be measured. Inshort, these methods are not commercially viable.

SUMMARY OF THE INVENTION

The present invention seeks to provide a soot monitoring device whichcontrols signal attenuation to levels which can be measured byconventional and relatively inexpensive electronic systems. To meet thisobjective, the present invention provides an antenna system which doestwo things. First, it restricts the transmission loss measurement toonly a fraction of the filter volume, thereby reducing the amount ofsoot in the RF signal path, and, second, the geometry of the antennasystem is arranged such that it does not require the filter housing toserve as a waveguide, thereby eliminating a number of the technicalproblems mentioned earlier and, thus, allowing the device to be used atfrequencies in the low MHz range. The lower frequencies mean lowersignal attenuation and lower device costs.

In its most basic form, the antenna system consists of paralleltransmitting and receiving antennae that are inserted parallel to thecentral axis of the cylindrical metal filter cavity and are inductivelycoupled in a direction radial to the antennae and filter axis. Theseantennae may be inserted in either end of the filter or both in the sameend of the filter. It can be readily demonstrated that the measurementvolume is axially confined to the area of overlap of the antennae and inthe radial direction by the metal walls of the filter housing. Eachantenna may consist of one or more metallic elements. The addition ofmore than one element to an antenna may be desirable in someapplications in order to improve the broadband frequency transmissionand reception characteristics of the antenna system. The antenna systemdesign geometry is closely coupled to the geometry of the filter. Thatis, the antenna geometry is adjusted to optimize transmission andreception of a selected frequency range within a specific filter systemgeometry.

The present invention is generally defined as an improved antenna systemfor an apparatus for detecting the accumulation of particulate materialon a filter medium formed of dielectric material and disposed in achamber, the apparatus being operable for generating and transmitting aRF signal through the filter medium and for monitoring the transmissionloss of the signal through at least a portion the filter medium so as toprovide an indication of the content of particulate material accumulatedon the filter medium, the improvement comprising an input antenna fortransmitting the signal and having at least one antenna elementextending longitudinally of the chamber and disposed within the filtermedium; and an output antenna for receiving the signal transmitted bythe input antenna and having at least one antenna element extendinglongitudinally of the chamber and disposed within the medium inparallel, spaced apart, axially overlapping relation with respect to theinput antenna element.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings wherein:

FIG. 1 is a diagrammatic, gross-sectional view of a diesel exhaustparticulate trap or filter adapted for RF detection of the sootaccumulation and a block diagram of an electrical circuit for carryingout the method of the present invention; and

FIG. 2 is a schematic of an electrical circuit in accordance with anembodiment of the present invention;

FIGS. 3a, 3b and 3c are top, bottom and side cross-sectional views ofone embodiment of the antenna system of the present invention in whichthe antenna system is characterized by a pair of inductively coupledmonopole antennae inserted into the exhaust inlet and outlet ends of awall-flow filter within a metal filter housing;

FIGS. 4a, 4b and 4c are top and side cross-sectional views of a secondembodiment of the antenna system of the present invention in which theantenna system is characterized by a pair of inductively coupled bipolarantennae are both inserted into either the exhaust inlet or exhaustoutlet end of a wall-flow filter within a metal filter housing;

FIG. 5 is a diagrammatic gross-sectional view illustrating the electricfield pattern within a filter cavity for a pair of inductively coupledmonopole antennae;

FIGS. 6 and 7 are schematic representations of the RF-measurement volumeassociated with the region of antennae axial overlap for two types ofantennae insertion methods;

FIG. 8 is a graph illustrating TEM mode transmission loss as a functionof frequency for a 30.48 cm long cylindrical waveguide; and

FIG. 9 is a graph illustrating RF transmission loss as a function offrequency;

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 illustrates a steel, cylindrical filter housing 10 havingfrusto-conical steel inlet and outlet end sections 12 and 14 adapted tobe connected to engine exhaust pipes in a manner well known in the art.The housing is formed with a chamber 15 to receive a ceramic filterelement 16 of suitable construction. A first probe 20 which behaves as atransmitting antenna for RF power and a second probe 22 which behaves asa receiving antenna for RF power are disposed within the housing andembedded within the filter element in a manner which is explained morefully below. A modulator 24 generates an amplitude modulated tone signalwhich is fed to an RF source 26 which, in turn, generates a carriersignal for the tone signal and applies the resulting signal to asplitter 28. Splitter 28 applies the signal to both transmitting probe20 and a first detector 30. Detector 30 produces a reference outputsignal which is representative of the power of the signal prior totransmission. The use of an amplitude modulated signal allows the signalto be much more easily detected than by the method used in theaforementioned General Motors Corporation patent.

A second detector 32, electrically connected to the second probe,produces an output signal representative of the power of the signalreceived by the second probe 22. The first and second detector outputsignals are applied to a comparator 34 which produces an output signalwhich is proportional to the difference in the signal strength of thetransmitted and received signals. Accordingly, the comparator outputsignal is representative of the transmission loss through the filtermedium which, in turn, is representative of the change in the effectivedielectric loss factor caused by accumulation of soot on the filter. Itwill be seen therefore that when there is little or no accumulation inthe filter, there will be only a small transmission loss in the signalstrength. As the soot accumulation increases, the difference in signalstrength between the transmitted and received signals changes, resultingeventually in an output signal from the comparator. The comparator canbe designed to drive a variable output display or an indication when apredetermined level is reached, or both.

The power source is arranged to emit RF energy over a range offrequencies with the preferred frequency band being up to one octave,i.e. a 2 to 1 range, in frequency. An appropriate frequency band is 150MHz to 250 MHz. There are three reasons for this. First, the averagetransmission loss through the filter over the selected frequency rangeresults in better measurement sensitivity, i.e. attenuation per unit ofsoot present, and a more linear response as a function of RF signalattenuation than is possible at a single frequency. Second, it avoidsproblems associated with power source frequency drift with time. Third,the use of an averaging process demonstrably reduces the effects oftemperature on transmission losses, i.e. the effects of temperature onsoot and filter permittivity, which would otherwise require temperaturecompensation in single or narrow band frequency methods.

With reference to the circuit diagram illustrated in FIG. 2, modulator24 will be seen to be comprised of an operational amplifier 50 which,with resistors 52, 54 and 56 and capacitors 58, 60 and 62, forms a phaseshift audio oscillator which provides a tone modulated signal along line64. This signal is fed via capacitor 66 to the gate of a FET modulatortransistor 68 which directly modulates the power supply to a frequencyswept RF source 70, thereby imposing an AM audio tone on the RF signaloutput along line 72. Resistors 74 and 76 form the gate bias network fortransistor 68. Resistors 80, 81 and 84, capacitor 86 and operationalamplifiers 88 and 90 form a sawtooth waveform sweep generator 91 whichfeeds a swept output signal to the frequency control port 94 of the RFsource so as to cause the RF oscillator output to vary by up to oneoctave in frequency. The sweep rate is set by resistor 80 and capacitor86. The output of the RF source is applied to splitter 18 which issimply comprised of a resistor 100 in series with resistors 101 and 104,respectively. The output of resistor 102 is fed to the transmit antennaor probe 20 while the output of resistor 104 is fed to the input ofreference detector 30. For equal power division, the resistances of thethree resistors are equal. The values of the resistances may be variedso that match is preserved with the system impedance but with most ofthe power passed to the soot filter.

Reference detector 30 and the signal detector 32 may be of identicalconstruction as indicated by subcircuits 110 in FIG. 2. Each circuit 110includes a capacitor 112 which provides DC isolation from alow-resistance source for a voltage-doubler signal detector 114comprised of diodes 116 and 118. Resistors 120 and Capacitor 122 providea level enhancing time constant for the detected modulation tone.Inductor 124 and capacitor 126 form a parallel tuned circuit at the tonefrequency which curtails the passband and improves the signal to noiseratio. Capacitor 128 prevents inductor 124 from shorting resistor 120.Operational amplifier 130 amplifies the signal tone by about 30 dB.Diode 132 rectifies the amplified tone signal to DC, with capacitor 134and resistor 136 setting the time constant and capacitor 138 andresistor 140 serving as a ripple filter. Each of the two detectors feeda respective input to the comparator.

Comparator 34 is formed with two sections generally designated byreference numerals 150 and 152. The reference detector output is feddirectly to the negative input of the second section 152 and indirectlyto the positive input of the first section 150 through a potentiometer154. Similarly, the signal detector output is fed directly to thenegative input of the first section 150 and indirectly to the positiveinput of the second section 152 through a potentiometer 156. Thepotentiometers serve to set the input levels from the signal andreference detectors to the two sections of the comparator. Morespecifically, in one section, potentiometer 154 sets its input below theoutput signal of the signal detector. As the signal level declines withincreasing soot, a point is reached where the negative input to thissection drops below the positive input and the output of the section isthen pulled up by resistor 158. In the other section, potentiometer 156is set so that the positive input is above the reference detector outputonly when the soot filter is clean. This serves as an optional check onthe bum-clean cycle. With the signal above the reference detector,resistor 160 pulls up this output. The outputs are connected toindicator circuits not shown.

FIGS. 3-5 illustrate two embodiments of an antenna system for use in awall-flow filter system, although it is to be understood that theantenna system be used with other filter types, such as ceramic foamfilters and the like, and/or other geometries, without departing fromthe spirit of the invention. The antenna system has been designed toprovide a soot monitoring device which reduces signal attenuation tolevels which can be measured by conventional and relatively inexpensiveelectronic systems. To that end, the antenna system does two things.First, it restricts the transmission loss measurement to only a fractionof the filter volume, thereby reducing the mount of soot in the RFsignal path. In the aforementioned United States patent, the RF signalmust propagate through the entire axial length of the filter. Thisrequires more power, more complex circuitry and more expense. Second,the geometry of the antenna system is arranged such that it does notrequire the filter housing to serve as a waveguide. This eliminates anumber of the technical problems mentioned earlier, particularly thoseassociated with the cutoff frequency. Thus, the device can be used atfrequencies in the low MHz range, well below the cutoff frequency of thesame housing used as a waveguide. The lower frequencies mean lowersignal attenuation and lower device costs.

In its most basic form, the antenna system consists of paralleltransmitting and receiving antennae that are inserted into the filtermedium, parallel to the central axis of housing. The antennae may beinserted into either end of the filter or both in the same end of thefilter. The antennae are inductively coupled in a radial direction withrespect to the antennae and the filter axis. It can be readilydemonstrated that the measurement volume is axially confined to the areaof overlap of the antennae and radially confined by the metal walls ofthe filter housing. Each antenna may consist of one or more metallicelements. The addition of more than one element to an antenna may bedesirable in some applications in order to improve the broadbandfrequency transmission and reception characteristics of the antennasystem. The antenna system design geometry is closely coupled to thegeometry of the filter. That is, the antenna geometry is adjusted tooptimize transmission and reception of a selected frequency range withina specific filter system geometry.

In the embodiment of FIG. 3, the antenna system is comprised of a pairof inductively coupled monopole antennae inserted into opposite ends ofa wall-flow filter disposed within a metal filter housing. The twoelements are parallel to one another and the axis of the filter elementbut axially overlap one another, as best shown in FIG. 6, to define anRF-Measurement volume 200. In the embodiment of FIG. 4, the antennasystem is comprised of a pair of inductively coupled bipolar antennaeinserted into the same end, either the inlet or the outlet end, of awall-flow filter disposed within a metal filter housing. The twoelements are parallel to one another and the axis of the filter elementand axially overlap one another, as best shown in FIG. 7, to define anRF-Measurement volume 200.

The antenna elements of the two illustrated embodiments are secured tothe RF feed-through-fittings and embedded in the filter element in thesame manner, as described hereinafter. Transmitting antenna 20 and areceiving antenna 22 extend through the wall of the vessel, through theRF feed-through-fittings 180, and penetrate the filter medium. Eachantenna includes a straight conductor portion 182 which passes throughits associated fitting and a pair of laterally spaced arms 184 and 186connected together by a semi-circular portion 188. Each arm is generallyL-shaped with a radial or transverse portion 190 and an axial portion192. As shown in the drawings, the axial portion penetrates the filtermedium. For illustration purposes, for a filter containment vessel witha length and diameter of 30.5 cm, the lateral spacing between antennaarms may be 40 mm, the radial portion of the arms may be 90 mm, theaxial length of arm penetration into filter may be 120 mm, the offset ofthe semi-circular portion from filter containment wall may be 20 mm andthe radius of curvature of semi-circular portion may be 20 mm. It willbe obvious to those skilled in the an that there are variations on theabove which might be better or equally good insofar as antennaperformance is concerned. It will also be understood that while two armshave been shown, improved performance might be achieved by the additionof a third arm to the antenna.

By adding to the number and/or modifying the position and/or length ofthe various metallic antenna elements, it should be understood that boththe size and/or the shape of the filter volume sampled by the antennasystem may be modified to suit various measurement requirements.

FIG. 8 plots the attenuation through the length of the filtercontainment for a 30.5 cm D×30.5 cm L filter. The actual path length islonger for an antenna at each length because it is not always possibleto place the antenna at the filter face and, hence, the actualtransmission losses shown in FIG. 8 are conservative. The graph ignoresother losses in the filter (i.e., filter and/or soot).

If it was desired to use an RF source in the 200 to 300 MHz range withconventional antenna methods, i.e., end-to-end transmission, in a 30.5cm diameter by 30.5 cm long wall-flow filter system, the totaltransmission loss would be estimated as follows. The (measured)transmission loss of a filter loaded to approximately 5 g/L is about 30dB. From FIG. 8, the transmission loss due to frequency cutoff at 300MHz is about 30 dB. Thus, the total transmission loss of a loaded filteris 30+30=60 dB. This mount of transmission loss is clearly outside therange, normally 20 to 30 dB, of a practical industrial device. FIG. 9shows the transmission loss as a function of frequency for a comparablesystem constructed according to the present invention. Losses in therange of 200 to 300 MHz are around 3 dB--an order of magnitudeimprovement over convention antenna methods.

It will be understood that various modifications and alterations may bemade to the present invention without departing from the spirit of theappended claims.

The embodiments of the invention in which an exclusive property ofprivilege is claimed are defined as follows: PROPERTY OF PRIVILEGE ISCLAIMED ARE DEFINED AS FOLLOWS:
 1. In an apparatus for detecting theaccumulation of particulate material on a filter medium, the filtermedium formed of dielectric material and disposed in a chamber, saidapparatus having means for generating and transmitting an RF signalthrough said filter medium and for monitoring the transmission loss ofsaid signal through at least a portion of said filter medium so as toprovide an indication of the content of particulate material accumulatedon said filter medium, the improvement comprising:an input antenna fortransmitting said signal and having at least a first antenna elementextending longitudinally within said chamber and disposed within saidfilter medium; and an output antenna for receiving the signaltransmitted by said input antenna and having at least a second antennaelement extending longitudinally within said chamber and disposed withinsaid medium in parallel, spaced apart, axially overlapping relation withrespect to said first antenna element.
 2. The apparatus as defined inclaim 1, wherein said input antenna and output antenna are inserted intoopposite ends of said filter medium.
 3. The apparatus as defined inclaim 1, wherein said input antenna and said output antenna are insertedinto the same end of said filter medium.
 4. The apparatus as defined inclaim 1, wherein said first antenna element having a first arm portionlaterally spaced from a second arm portion of the second antenna elementand a first connecting portion at an end of said first arm portion forconnecting with a second connecting portion of the second antennaelement.
 5. The apparatus as defined in claim 4, wherein each of saidinput antenna and output antenna has a conductor portion which passesthrough a wall of said chamber, and one end of said conductor portion isconnected to a connecting portion of each said antenna element withinsaid chamber.
 6. The apparatus as defined in claim 1, wherein said inputantenna transmits said RF signal at frequencies in the lower MHz rangethan the cutoff frequency of said chamber.
 7. The apparatus as definedin claim 6, wherein said input antenna transmits said RF signal atfrequencies in the range of 150-250 MHz.